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Distillation separating, or distillation, is a physical separation process that exploits differences in volatility among components of a liquid mixture. By heating the mixture, more volatile components tend to vaporize first; the vapor is then cooled and condensed into a liquid, yielding fractions with distinct composition. The process can be used to purify a single compound or to separate a multicomponent mixture into several fractions.

In simple distillation, a single stage of vaporization and condensation occurs, and separation quality depends on the boiling point difference of the components. Fractional distillation improves this separation by using a distillation column packed with or containing trays, which creates many theoretical plates where vapor and liquid equilibrate. A higher relative volatility between components and an appropriate reflux ratio lead to sharper separations and purer fractions. Key parameters include boiling points, relative volatility, column height or packing, and the reflux ratio.

Variants of distillation address specific challenges. Vacuum distillation lowers boiling points to prevent thermal decomposition of heat-sensitive substances. Steam distillation uses steam to carry volatile compounds at reduced temperatures, common in essential oil and fragrance production. Azeotropic and extractive distillation are techniques to handle mixtures that form azeotropes or otherwise resist separation by conventional means, using entrainers or solvents to alter relative volatilities.

Applications span the chemical and petrochemical industries, beverage production, purification of solvents, production of fuels and lubricants, and the isolation of natural products. Limitations include energy intensity, the difficulty of separating components with close boiling points or forming azeotropes, potential thermal degradation, and equipment costs.